1. Field of Invention
The invention relates to a method and system for locating an unknown transmitter. More particularly, the invention relates to a method and system for locating an unknown radio-frequency transmitter using calibrated oscillator phases.
2. Description of Related Art
Various techniques have been proposed for locating an unknown transmitter that causes interference at a satellite receiver. One such technique is described in U.S. Pat. No. 5,008,679 ("the '679 patent"), entitled "Method and System for Locating an Unknown Transmitter" issued to Effland et al. on Apr. 16, 1991, which is hereby incorporated by reference.
The '679 patent describes a method and system for determining the locations of unknown transmitters that serve as sources of interference to receivers, such as geosynchronous satellite communications systems, low-Earth-orbit satellites, aircraft, and other receivers of electromagnetic radiation. According to the '679 patent, at least two satellite receivers receive an interfering signal from an unknown transmitter and retransmit these signals to a receiving station. The receiving station receives and processes the retransmitted interfering signals through cross-correlation to determine the location of the unknown transmitter.
As discussed, the method described in the '679 patent preferably employs at least two satellite receivers. Each of these satellite receivers typically includes at least one transponder, which comprises a receiver, a frequency converter, and a retransmitter. The frequency converter will usually contain a local oscillator, which is phase-locked to a reference frequency source, such as a crystal oscillator.
The crystal oscillators on each of these satellites, however, are independent from each other. As a result, the frequencies or phases of the crystal oscillators, and thus those of the local oscillators, can drift unpredictably with respect to each other due to, for example, age, temperature, mechanical force, or power supply voltage. Such drift effectively limits the length of time that the signals remain sufficiently coherent in phase for a cross-correlation process to work, which, in turn, limits the strength of the detectable signal. Thus, it is desirable to compensate for drift between local oscillators in the satellite receivers.
One proposed method for compensating for such drift is described in a report entitled "Field Trials of a Transmitter Location System Using INTELSAT Satellites," by John E. Effland et al., INTEL-874 Phase 3, Dec. 30, 1991 (hereinafter "the INTELSAT report"). According to this proposed method, in addition to the interfering signal, the two satellites also receive a phase-calibrating signal from a known transmitter and retransmit this signal to the receiving station. The receiving station receives the retransmitted phase-calibrating signals separately from the interfering signals and processes them independently from the interfering signals to obtain a compensation signal. This compensation signal is then used to correct inaccuracies in the retransmitted interfering signals resulting from discrepancies in phase or frequency between the oscillators of the satellites.
This proposed method, however, is less than ideal because only two signals are processed at the same time, namely the unknown signal and the compensation signal. If the compensation signal is degraded in some way due to signal propagation, momentary antenna mispointing, or other effects, then the entire set of data obtained must be discarded. Furthermore, it is also necessary to observe one or more additional signals from known positions in order to correct any inaccuracies in the satellite orbits. This means that observations of the unknown signal must be interleaved with observations of other calibrating signals, which degrades both the accuracy and the speed with which the position of the unknown transmitter can be determined. This fact is due to the unpredictable nature of the local oscillator phase changes and unmodeled variations in the satellite orbits. The amount of time required for a satisfactory set of data to be obtained is thus greater than would be the case if all signals of interest were observed simultaneously.
In the method described in the INTELSAT report, such a mode of obtaining data would require large increases in the expense of the equipment because separate signal paths and correlators would be required for each signal. Furthermore, since each signal is observed with a separate set of equipment, instrumental effects within the separate sets of equipment may result in unremovable systematic errors in the data. Thus, it is desirable to perform the frequency compensation more rapidly and at minimum expense by observing all signals of interest simultaneously with one set of equipment in one correlation process.